The invention relates to a method of fabricating a light image detector and a linear detector obtained by this method. It relates to the construction of thin film electronic circuits over large areas.
Its application is to be found more particularly in contact facsimile devices without reduction optics.
Since the discovery of the high photoconductivity of amorphous silicon and the ability to deposit it over a large area, numerous applications have been studied and in particular large scale detectors for compact office facsimile machines. Researchers have turned towards integration of the photoconducting elements associated with a blocking electronic device (for blocking the interferences between the photoconductors and reading switches).
The application concerned by the present invention is the integrated control of each elementary point of a light image detector.
At the present time, the main applications associating the high photoconductivity of amorphous silicon with the possibility of forming control electronics in this same material and on the same support relate to linear image detectors, of a large size (up to 20 cm), required for reading A4 format documents by lateral movement of the document with respect to the sensor.
Three different structures have been contemplated for forming such linear detectors,
the association of a photoconductor, a storage capacity and a thin film transistor (TFT) made from amorphous silicon (a-Si) such as is described in the article by M. MATSUMURA published in the IEEE review Electron Device Letters, EDL-1, p. 182 (1980); PA1 the association of an amorphous silicon or polycrystalline photodiode such as described in "Extended Abstracts of the 16th Conference on Solid State Devices and Materials", TOKYO (1983), p. 201 to 204 by F. OKUMURA; PA1 the association of a photodiode and an a-Si blocking diode such as described by Y. YAMAMOTO in "Extended Abstracts of the 15th Conference on Solid State Devices and Materials" Tokyo (1983), p. 205 to 208. PA1 (a) a first phase of depositing on a face of the substrate a first layer of a conducting material; PA1 (b) a second phase of depositing a first doped amorphous semiconductor layer of a first given type; PA1 (c) a third phase of depositing a second undoped amorphous semiconductor layer; PA1 (d) a fourth phase of depositing a third doped amorphous semiconductor layer of a second given type; PA1 (e) a fifth phase of depositing a fourth undoped amorphous semiconductor layer; PA1 (f) a sixth phase of depositing a fifth doped amorphous semiconductor layer of the first given type; PA1 (g) a seventh phase of etching columns in the five previously deposited semiconductor layers, as well as in the first conducting material layer; PA1 (h) an eighth phase of etching individual detect in the five semiconductor layers of the preceding columns; PA1 (i) a ninth phase of isolating the sides of the individual detectors; PA1 (j) a tenth phase of depositing a second layer of a conducting material; PA1 (k) and an eleventh phase of etching in the second conducting material layer electrical connections to the individual detectors.
Some of these structures allow devices to be obtained having the following characteristics:
______________________________________ Length 50 mm Resolution 8 to 10 bits/mm Size of the pixel 100 .mu.m .times. 70 .mu.m Reading time 2 .mu.s/bit without multiplexing. ______________________________________
It can be assumed from these results that linear detectors will be formed in the near future for reproducing documents under good conditions. For example, for documents of 216 mm in width, at the rate of 8 bits per millimeter and at a travelling speed over the page of 5 ms per line.
An A4 format (216 mm) strip must then represent 1728 photoconductor pairs plus switches so as to have 8 bits/mm or 3456 pairs so as to have 16 bits/mm in the future tendency.
It can be readily understood that such a high number of electronic devices over such a distance requires the simplest structure and technology. This is the first requirement and in this sense the complex structure of thin film transistors (TFT) is not very appropriate.
The second requirement relates to the rapid reading mode of the document, namely about 2 .mu.s/bit Hereagain diodes with high speed photoresponse and switching times appear better adapted than thin film transistors.
The third requirement is integration on the same substrate of the connecting matrix circuit so as to reduce the number of hybrid control circuits and this with the lowest possible number of masking levels.
A last requirement is to elaborate high performing electronic devices having a very low dark current.
The object of the invention concerns a linear image detector associating at each point the photodiode and an amorphous silicon blocking diode and superimposed with integration of the matrix circuit.
According to the invention, Schottky or pin diodes are well adapted especially if the amorphous silicon layers forming the diodes are deposited successively and naturally on the same substrate.
The requirements enumerated above are respected with the advantage that the method of the invention allows such a detector to be obtained with a reduced masking level.